Spread spectrum techniques are widely used for wireless communication. Current applications of spread spectrum technology include systems for cellular telephony, systems for cellular data transfer, systems for communications between satellites and ground stations, and wireless local-area networks. One common implementation of spread spectrum technology is code division multiple access (CDMA) signal modulation. A CDMA system may be designed to support one or more CDMA standards such as (1) the “TIA/EIA-95-B Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System” (the IS-95 standard), (2) the “TIA/EIA-98-C Recommended Minimum Standard for Dual-Mode Wideband Spread Spectrum Cellular Mobile Station” (the IS-98 standard), (3) the standard offered by a consortium named “3rd Generation Partnership Project” (3GPP) and embodied in a set of documents including Document Nos. 3G TS 25.211, 3G TS 25.212, 3G TS 25.213, and 3G TS 25.214 (the WCDMA standard), (4) the standard offered by a consortium named “3rd Generation Partnership Project 2” (3GPP2) and embodied in a set of documents including “TR-45.5 Physical Layer Standard for cdma2000 Spread Spectrum Systems,” the “C.S0005—A Upper Layer (Layer 3) Signaling Standard for cdma2000 Spread Spectrum Systems,” and the “C.S0024 CDMA2000 High Rate Packet Data Air Interface Specification” (the CDMA2000 standard), and (5) other standards.
In a typical CDMA system, data traffic such as voice and/or other information is exchanged over a wireless link between a base station (also called a base transceiver station or BTS) and a user equipment (UE). The BTS may also be configured to exchange the information with one or more networks such as a public switched telephone network (PSTN) and/or a packet data switched network (PDSN). A BTS may communicate with other BTSs and/or controllers to support activities such as handoff of a communications session with a mobile UE. Communications between the BTS and a network or other BTS may be performed over wired and/or wireless connections.
A UE may include a receiver embodied in a cellular telephone, such as a mobile telephone or a fixed installation as may be found in a wireless local loop (WLL). Typically the receiver is included in or integrated into a transceiver of the UE. The receiver may include, or may be embodied in, a chip or chipset of the UE. The UE may be part of another mobile or portable unit, such as a wireless data modem or other peripheral device, a personal digital assistant (PDA), or a global positioning system (GPS) device. The UE may also include or be a part of a camera or another multimedia device such as a video or MP3 player.
Several instances of a single transmitted spread-spectrum signal may reach the receiver, each instance corresponding to a different propagation path and arriving at a different time. Receivers or transceivers for spread-spectrum signals typically use a rake receiver architecture to process more than one received instance of a transmitted signal. FIG. 1A shows an example of a rake receiver having multiple fingers (also called “demodulation elements” or “finger processors”), in which each finger outputs an estimate of the received symbol that corresponds to a different multipath instance of the signal. After deskewing to compensate for the relative time delays (or “skew”) between the instances, a combiner adds the various symbol estimates together to generate a combined symbol estimate.
The particular multipath instance that each finger tracks may be determined autonomously by the finger, cooperatively among the fingers, and/or according to an assignment received by the finger from a searcher. FIG. 1B shows an example of a rake receiver architecture that includes a searcher. The searcher determines the locations in time of peaks in the received multipath signal and assigns each finger to a path according to the time offset of the corresponding peak. The searcher may provide an offset value to the finger, indicating the delay of the assigned path in terms of code phase, or the searcher may supply a version of the spreading code that is offset according to the delay of the assigned path.
As symbol estimates corresponding to more received instances of the same symbol are combined, it may be expected that the signal-to-noise ratio (SNR) of the combined estimate will be improved. Thus it is generally desirable to combine symbol estimates corresponding to several different received instances if possible. Some received instances may be weak, however, such that they contain more noise energy than signal energy. Including an estimate from such an instance may reduce the overall SNR, and in these cases it may be desirable to exclude the estimate from the combined symbol estimate.